This invention relates to the production of silicon. More particularly, this invention relates to the in situ growth of large silicon crystals on a substrate suitable for use in the manufacture of solar cells.
The production of solar cells using silicon has lead to much investigation into more economical methods of preparing the crystalline silicon used in the cell.
Conventionally, single crystal boules of semiconductor grade silicon are grown, sliced into wafers, polished, and then mounted on substrates prior to doping and subsequent processing to form a solar cell.
The costs of crystal growing, slicing, polishing, and mounting have resulted in experimentation with methods for forming the crystals directly on the substrate including chemical vapor deposition (CVD) of silicon on a substrate. While such CVD deposition does result in the formation of silicon crystals having the requisite purity, the size of the crystals has typically been below 10 microns. Such techniques have, therefore, not been adopted for production of solar cells because the efficiency of the cell is related to the size of the silicon grains or crystals. A minimum crystal size deemed acceptable for production of an efficient solar cell has been determined to be about 100 microns.
Attempts have been made to alter the size of the deposited silicon crystals resulting from CVD grown polycrystalline silicon. M. W. M. Graef, L. J. Giling, and J. Bloem published a paper entitled "Chemical Vapor Deposition of Silicon On A Liquid Tin Layer" in the Proceedings of the photovoltaic Energy Conference, Luxembourg, 1977, in which they described the effect of chemical vapor deposition (CVD) of silicon by pyrolysis of silane or trichlorosilane in a hydrogen atmosphere onto a substrate covered with a liquid tin film. The authors described the growth of silicon crystals having grain sizes over 100 microns by this method. In a later paper by the same authors and H. H. C. De Moor entitled "The Growth of Polycrystalline Silicon Layers on Top of a Tin-Coated Substrate: In Situ Observations", published in the Proceedings of the 2nd Photovoltaic Solar Energy Conference, Berlin, 1979, it was further noted that the liquid tin substrate could be provided by vacuum deposition of tin on a graphite substrate or by the reduction of SnCl. The authors stated that the surface tension was subsequently reduced by the injection of three pulses of trichlorosilane to form a continuous liquid layer. The deposition of silicon on the liquid tin surface was said to be accomplished by the continuous addition of 0.75% SiHCl and 1% HCl in H causing the growth of needle- and plate-like shaped silicon islands. This method was further described in an article by W. J. P. van Enckevort and M. W. M. Graef entitled "Growth Mechanisms of Silicon Crystallites Grown on Top of a Metal-Coated Graphite Substrate", published in Volume 128 of the Journal of the Electrochemical Society: Solid State Science and Technology in January, 1981. In this later article the HCl is described as causing the evaporation of the liquid tin substrate by the etching activity of the HCl.
While the foregoing articles describe a method which apparently results in the in situ production of silicon crystals of a satisfactory size for solar cell use, the added step of preparing a molten metal coating on a substrate somewhat precludes consideration of the approach from an economical standpoint.
There, therefore, remains a need for a method for the in situ production on a substrate of silicon crystals sufficiently large to be useful in the production of a solar cell therefrom.